RFE: Support OnlySubCommands arg validation to make subcommand get suggestions

[nak3]

Motivation

Currently, suggestions for "unknown command" are not working for
subcommands. For example code[1](wants to accept only when
subcommand specified), I expected that cobra give us suggestions like:

Expected

  $ go run main.go echo time
  Error: unknown command "time" for "echo"

  Did you mean this?
   	times

However, cobra does not suggest anything unfortunately.
SuggestFor: attribute also does not work currently.

Workaround

If I use OnlyValidArgs for the subcommands, this could be
solve. However, it does not suit when a command has many subcommands
and makes it makes difficult to maintain.

Proposal patch

#635

[1] example code

package main

import (
        "fmt"
        "strings"

        "github.com/spf13/cobra"
)

func main() {
        var echoTimes int
        var cmdEcho = &cobra.Command{
                Use:   "echo",
                Short: "My root command",
                Run: func(cmd *cobra.Command, args []string) {
                        fmt.Println("Don't want to call this...")
                },
        }

        var cmdTimes = &cobra.Command{
                Use: "times [# times] [string to echo]",
                Run: func(cmd *cobra.Command, args []string) {
                        for i := 0; i < echoTimes; i++ {
                                fmt.Println("Echo: " + strings.Join(args, " "))
                        }
                },
        }
        cmdTimes.Flags().IntVarP(&echoTimes, "times", "t", 1, "times to echo the input")
        var rootCmd = &cobra.Command{Use: "echo"}
        rootCmd.AddCommand(cmdEcho)
        cmdEcho.AddCommand(cmdTimes)
        rootCmd.Execute()
}

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